This invention relates to a process for producing hydrocarbons useful as fuel, such as aviation fuel, from renewable feedstocks with the glycerides and free fatty acids found in materials such as plant oils, fish oils, animal fats, and greases. The process involves hydrogenation, decarboxylation, decarbonylation, and/or hydrodeoxygenation, hydroisomerization, and selective cracking in two or more steps. The selective cracking step optimally provides one cracking event per molecule. A reforming step may be optionally employed to generate hydrogen used in the hydrogenation, deoxygenation, hydroisomerization, and selective hydrocracking steps.
As the demand for fuel such as aviation fuel increases worldwide there is increasing interest in sources other than petroleum crude oil for producing the fuel. One such source is what has been termed renewable sources. These renewable sources include, but are not limited to, plant oils such as corn, rapeseed, canola, soybean and algal oils, animal fats such as tallow, fish oils and various waste streams such as yellow and brown greases and sewage sludge. The common feature of these sources is that they are composed of glycerides and Free Fatty Acids (FFA). Both of these classes of compounds contain aliphatic carbon chains generally having from about 8 to about 24 carbon atoms. The aliphatic carbon chains in the glycerides or FFAs can be fully saturated, or mono-, di- or poly-unsaturated.
There are reports disclosing the production of hydrocarbons from oils. For example, U.S. Pat. No. 4,300,009 discloses the use of crystalline aluminosilicate zeolites to convert plant oils such as corn oil to hydrocarbons such as gasoline and chemicals such as para-xylene. U.S. Pat. No. 4,992,605 discloses the production of hydrocarbon products in the diesel boiling range by hydroprocessing vegetable oils such as canola or sunflower oil. Finally, US 2004/0230085 A1 discloses a process for treating a hydrocarbon component of biological origin by hydrodeoxygenation followed by isomerization.
Applicants have developed a process which comprises two or more steps to hydrogenate, deoxygenate, isomerize and selectively crack a renewable feedstock, in order to generate a fuel such as aviation fuel. Simply deoxygenating the renewable feedstock typically results in strait chain paraffins having chain-lengths similar to, or slightly shorter than, the fatty acid composition of the feedstock. With many feedstocks, this approach results in a fuel meeting the general specification for a diesel fuel, but not for an aviation fuel. The selective cracking step reduces the chain length of some paraffins to maximize the selectivity to aviation fuel range paraffins while minimizing light products. The selective cracking may occur before, after, or concurrent with the isomerization. An optional reforming step may be included to generate the hydrogen needed in the deoxygenation and the isomerization steps. In one embodiment, a recycle from the effluent of the deoxygenation reaction zone back to the deoxygenation zone is employed. The volume ratio of recycle hydrocarbon to feedstock ranges from about 2:1 to about 8:1 and provides a mechanism to increase the hydrogen solubility and more uniformly distribute the heat of reaction in the deoxygenation reaction mixture. As a result of the recycle, some embodiments may have a lower operating pressure.